1. Field of the Invention
The present invention is generally in the field of semiconductors. More particularly, the invention is in the field of field effect transistor (FET) fabrication.
2. Background Art
In CMOS technologies, such as deep submicron CMOS technologies, boron difluoride (BF2) is typically implanted to form P type polycrystalline silicon (poly) gates and P type source/drain regions for P-channel field effect transistors (PFETs). Incorporation of fluorine in the polysilicon gate and source/drain region implant provides various advantages, such as a shallow P type junction depth, improved device radiation hardness, and lower device interface degradation from hot electron effects. However, when titanium silicide, which is utilized for its low sheet resistance and low contact resistance to silicon, is formed in poly gates that have been implanted with BF2, fluorine that dissociates from the implanted BF2 can form voids and/or bubbles in the titanium silicide, which can cause increased sheet resistance and large sheet resistance variations in the titanium silicide and, consequently, increased poly gate resistivity.
Various conventional approaches have been attempted to overcome the above discussed problems caused by fluorine on titanium silicide sheet resistance and, consequently, on poly gate resistivity. One conventional approach is to use boron in place of BF2 for P type implanting of PFET poly gates. Although titanium silicide formed in boron implanted polysilicon does not have the sheet resistance problems of titanium silicide formed in BF2 implanted polysilicon, boron has a faster diffusion rate than BF2, which can undesirably affect transistor performance. Another conventional approach is to form a layer of silicon oxide on the BF2 implanted poly gate before forming the titanium silicide. In this approach, an anneal can be performed to cause the fluorine in the BF2 to diffuse into the silicon oxide layer, which can be removed prior to titanium silicide formation. However, this approach can undesirably affect transistor design and can also undesirably increase process complexity.